The present invention relates to an image forming method for a printer, copier facsimile apparatus or similar image forming apparatus.
To better understand the present invention, conventional technologies relating to image formation will be described first.
Japanese Patent Laid-Open Publication No.10-177286 (prior art 1 hereinafter) contemplates reducing the size of an image forming apparatus, increasing the number of images to be formed for a unit period of time, and reducing the number of processing units. Specifically, prior art 1 pertains to an image forming apparatus of the type transferring a color image from an intermediate image transfer belt to a recording medium with image transferring means. The apparatus includes first and second image forming units spaced from each other along the belt. The first image forming unit includes a single photoconductive drum and two developing means each for developing a particular latent image formed on the drum with toner of color A or B. Likewise, the second image forming unit includes a single photoconductive drum and two developing means each for developing a particular latent image formed on the drum with toner of color C or black toner.
Japanese Patent Laid-Open Publication No. 11-109708 (prior art 2 hereinafter) proposes an image forming apparatus of the type including two image stations arranged around an intermediate image transfer body. The image stations each include a respective photoconductive element and two developing means facing the photoconductive element. At each image station, the developing means are switched to form toner images of different colors on the photoconductive element. The toner images are sequentially transferred to the intermediate image transfer body one above the other. The resulting color image is transferred from the image transfer body to a recording medium. In accordance with prior art 2, each image station includes a single driveline for driving the two developing means and switching means for selectively transmitting the drive of the driveline to either one of the two developing means.
Japanese Patent Laid-Open Publication No. 11-125968 (prior art 3 hereinafter) discloses an image forming apparatus of the type including a rotatable image carrier and two developing means adjoining each other while facing the outer circumference of the image carrier. A developing function is switched from one developing means to the other developing means while the image carrier is in rotation, so that latent images are sequentially developed in two different colors. To provide a period of time necessary for switching the developing means, prior art 3 starts development with upstream one of the developing means in the direction of rotation of the image carrier and then starts development with downstream one of the developing means.
Japanese Patent Laid-Open Publication No. 11-218974 (prior art 4) discloses a device for image quality compensation that executes, based on the density of a test patch image, image quality control in accordance with the condition of an image to thereby maintain preselected image quality. Specifically, the device senses at least the density of the edge of an image where density is high and that of a center portion where density is stable. The device then sets an amount of exposure by comparing the sensed density of the high density portion and the condition of the image, e.g., the reference density of a line image. Also, the device controls the quantity of exposure by comparing the sensed density with, e.g., the reference density of a halftone image or similar solid image. In this manner, the device executes image quality compensation with a single test patch image in accordance with the condition of an image. Prior art 4 describes in paragraph xe2x80x9c0047xe2x80x9d that it usually executes the image quality compensation control before the start of image formation, e.g., on the power-up of an image forming apparatus or when the apparatus is not operating.
Japanese Patent Laid-Open Publication No. 11-218696 (prior art 5 hereinafter) teaches a multicolor image forming apparatus capable of preventing the quality of an image printed on a recording medium and output speed from falling. The apparatus forms test patterns of different colors for positional shift detection on a primary image transfer body during intervals between image formation. The apparatus reads the test patterns to determine the shift of write start positions in the subscanning direction and then varies the duty of a reference clock to be fed to a polygonal mirror, thereby controlling the rotation phase of the mirror. This is successful to correct the write start positions by controlling only the phase of the reference clock instead of frequency. Consequently, the variation of rotation of the polygonal mirror and therefore the mirror rotation control time is reduced.
Further, Japanese Patent Laid-Open Publication No. 11-2394 (prior art 6 hereinafter) discloses an image forming apparatus constructed to obviate image deterioration ascribable to fog toner deposited on the surface of an intermediate image transfer body without resorting to a cleaner. When the number of sheets fed in an A4 profile position reaches a preselected number, control means so controls a tray shift motor as to shift a sheet tray in the lateral direction. At the same time, the control means varies a position for starting forming a latent image in accordance with the position of sheet conveyance.
The conventional technologies described above have various problems left unsolved, as will be described hereinafter.
Prior art 4 usually executes image quality compensation control before the start of image formation, as stated earlier. In practice, however, it is likely that images are deteriorated even during image formation when a number of images are continuously output. It is therefore necessary to execute the above control even during image formation by sensing the densities of test patches.
Prior arts 1, 2 and 3 each include two image stations each having a respective intermediate image transfer body and two developing means arranged around the image transfer body. The process for forming toner images of different colors by switching the two developing means is executed with each of the two photoconductive elements. The resulting color images are transferred to the intermediate image transfer body one above the other and then to a sheet. In this case, the developing function is switched from the upstream developing means in the direction of rotation of the photoconductive element to the downstream developing means or from the latter to the former. The interval between the time when the trailing edge of an image developed by one developing means passes the developing means and the time when the leading edge of a latent image to be formed by the other developing means arrives at the other developing means differs between the above two different cases, as described in paragraph xe2x80x9c0019xe2x80x9d of prior art 3.
Prior art 5 pertains to control over image forming timing that detects a shift on the intermediate image transfer body by using test patterns. Prior art 5 describes in paragraphs xe2x80x9c0002xe2x80x9d through xe2x80x9c0005xe2x80x9d the purpose of image forming timing control and prior art control schemes based on test pattern images. Particularly, in paragraph xe2x80x9c0004xe2x80x9d, prior art 5 describes why image forming timing control based on the position of a test pattern during image formation is necessary.
Prior art 6 proposes a solution to the deterioration of images ascribable to fog toner. Particularly, in paragraph xe2x80x9c0007xe2x80x9d, prior art 6 describes specifically why images are deteriorated by fog toner when they are formed at a preselected position on the intermediate image transfer belt at all times. Further, in paragraphs xe2x80x9c0024xe2x80x9d through xe2x80x9c0029xe2x80x9d, prior art 6 describes that output images are counted and, when the count reaches preselected one, a plurality of home position sensors senses a mark formed on the intermediate image transfer body to thereby shift the image forming position on the transfer body. A problem with prior art 6 is that a controller must count output images and must control the image forming position, making the apparatus sophisticated and expensive. The plurality of sensors aggravates this problem. Another problem is that when the image forming position on the intermediate image transfer body is preselected, the image transfer body deteriorates more in the image portion than in the non-image portion. This prevents the life of the intermediate image transfer body from being extended.
It is therefore an object of the present invention to provide a method capable of promoting the high speed, small size, low cost configuration of an image forming apparatus in relation to image quality compensation control, which is executed during image formation by using test patches.
It is another object of the present invention to provide a method capable of promoting the high speed, small size, low cost configuration of an image forming apparatus in relation to image forming timing control, which is executed during image formation by using test pattern images.
It is a further object of the present invention to provide a method capable of extending the life of an intermediate image transfer body, obviating image deterioration ascribable to fog toner, and promoting the high speed, small size, low cost configuration of an image forming apparatus
In accordance with the present invention, an image forming method uses a plurality of image stations each including a single rotatable image carrier and two developing means each for developing a particular latent image formed on the image carrier in a respective color to thereby produce a toner image. The method switches a developing function from one developing means to the other developing means while the image carrier is in rotation, sequentially transfers toner images produced by the developing means to an intermediate image transfer body one above the other, and transfers the resulting color image from the intermediate image transfer body to a recording medium. A test patch image is formed on the image carrier at each image station after image formation using upstream one of the developing means in the direction of rotation of the image carrier or before image formation using downstream one of the developing means. Image quality compensation control is effected by sensing the density of the test patch image.